Jim,
On my 22, I tried a small ac unit in the front center window for a few trips. With almost loosing it overboard and dropping the GFI cord assembly in the water, while installing the ac (it was 98 degrees, high humidity and no wind) my wife convinced me to go to a rooftop unit. I would never turn back and wish my new 26 had a rooftop, vice built in ac...
The rooftop that I went with was the Coleman MACH 8, 9000 BTU unit. I also installed the heat strip and that is plug and play. The overhead of the 22 is solid glass and support nor trim was required to mount it. The unit came with a very thick piece of foam to mount the top of the ac on and I was able to cut the foam to the shape/contour of the overhead.
I chose the 9000 BTU over large units, or heat pumps, because I wanted to be able to power it, when not at a dock, with a small portable generator. On the hottest of days, we have found that the unit cooled the cabin down quickly and that the load on the ac was very light. Also, we have used the heater when temps outside near freezing and had to wake up the first few time that we used it to turn the thermostat way down, because we got too warm.